Dicarboxylic acids comprising six or more carbon atoms are commonly referred to as “long-chain diacids”. Long-chain diacids can be used as basic constituent monomer for a series of synthetic materials. Potential uses of long-chain diacids and their derivatives include, for example, production of special nylon resins, polycarbonate, powder coatings, fragrances, hot-melt adhesives and special lubricants. Long-chain diacids can also be used as plasticizers for engineering plastics and corrosion inhibitors in, for example, metal processing technology. When used as constituent monomers for production of special nylon, long-chain diacids can demonstrate some unique performance characteristics when compared to other monomers.
Commercial quantities of long-chain diacids are generally not found in nature. Certain long-chain diacids, for example adipic acid, sebacic acid and dodecanedioic acid, can be prepared via chemical methods. For example, starting with benzene or 1,3-butadiene, dodecanedioic acid can be prepared through multiple steps of chemical reactions. Sebacic acid can be prepared through the chemical conversion of castor oil. Starting with cyclohexane, adipic acid can be prepared through multiple steps of oxidation. Long-chain diacids can also be prepared via a biological method. A biological method, for example fermentation, can produce a series of long-chain diacids containing 6 through 18 carbon atoms. Some of the chemical and biological routes to diacids can result in low levels of analogous chain length monocarboxylic acids and/or hydroxyl acids as impurities. As these impurities can impact the suitability of the diacids in the desired applications, removal of the monocarboxylic acid and hydroxyl acid from the diacid is critical.
Chromatography, for example paper chromatography, gas chromatography, and high pressure liquid chromatography, can be utilized for identification and separation of long-chain diacids. The target long-chain diacid(s) and impurities have different interacting forces with the chromatograph stationary phase. Under specific eluting conditions and/or with a specific chromatograph stationary phase, the differences between the interacting forces could be large enough to achieve separation of different components. US20120253069 describes a laboratory method of using liquid chromatography with a packed bed column to separate long chain diacids from alkanes and other long chains diacids.
The process of separating a binary mixture is illustrated with reference to a single zone system as shown in FIG. 1. The concept of a simulated or actual continuous countercurrent chromatographic separation process is explained by considering a vertical chromatographic column containing stationary phase S divided into sections, more precisely into four superimposed sub-zones I, II, III and IV going from the bottom to the top of the column. The eluent is introduced at the bottom at IE by means of a pump P. The mixture of the components A and B which are to be separated is introduced at IA+B between sub-zone II and sub-zone III. An extract containing mainly B is collected at SB between sub-zone I and sub-zone II, and a raffinate containing mainly A is collected at SA between sub-zone III and sub-zone IV.
In the case of a simulated moving bed system, a simulated downward movement of the stationary phase S is caused by movement of the introduction and collection points relative to the solid phase. In the case of an actual moving bed system, downward movement of the stationary phase S is caused by movement of the various chromatographic columns relative to the introduction and collection points. In FIG. 1, eluent flows upward and mixture A+B is injected between sub-zone II and sub-zone III. The components will move according to their chromatographic interactions with the stationary phase, for example adsorption on a porous medium. The component B that exhibits stronger affinity to the stationary phase (the slower running component) will be more slowly entrained by the eluent and will follow it with delay. The component A that exhibits the weaker affinity to the stationary phase (the faster running component) will be easily entrained by the eluent. If the right set of parameters, especially the flow rate in each zone, are correctly estimated and controlled, the component A exhibiting the weaker affinity to the stationary phase will be collected between subzone III and sub-zone IV as a raffinate and the component B exhibiting the stronger affinity to the stationary phase will be collected between sub-zone I and sub-zone II as an extract.